The first step of viral infection is binding of virus to its target cells. Binding of HIV is usually mediated by viral envelope proteins, gp120, and its receptors, including CD4 and DC-SIGN. Therefore, previous studies of HIV binding have mainly focused on the interaction between gp120 and their receptors. We have recently found an envelope- protein independent manner of virus binding. One type of viral envelope lipid called phosphatidylserine binds to its receptors on target cells and mediates virus attachment. Since phosphatidylserine is specifically exposed on apoptotic cells, phosphatidylserine receptors are typically used for removal of apoptotic cells by phagocytes. We found that HIV replication is also drastically enhanced by one specific type of phosphatidylserine receptor, TIM-1, which is expressed on CD4-positive cells. In the proposed studies, we will investigate the molecular mechanism by which envelope phosphatidylserine and TIM-1 facilitates viral replication and whether CD300a, which is a newly identified phosphatidylserine receptor expressed on macrophages, can also support HIV replication. We will also investigate how HIV exposes phosphatidylserine on viral envelope. These studies will open a new avenue in HIV replication mechanism and help develop novel antiviral strategies that target phosphatidylserine-mediated viral replication.